1. Field of the Invention;
This invention relates to a liquid crystal display unit and more particularly it relates to improvements in the visibility of a display produced by a reflection type liquid crystal display unit.
2. Description of the Prior Art
FIG. 1 is a perspective view of a typical conventional, reflection type liquid crystal display unit shown disassembled. This liquid crystal display unit includes a liquid crystal display element 2. The liquid crystal display element 2 includes a first glass sheet 4 and a second glass sheet 6. A spacer 8 made, e.g., of Mylar (trade name) is interposed between the first and second glass sheets 4 and 6. As a result, a thin clearance of about 10 microns is defined between the first and second glass sheets 4 and 6. The clearance is filled with a liquid crystal 10. One surface of the first glass sheet 4, namely, the surface opposed to the second glass sheet 6 is formed with transparent segment electrodes 12 in a desired pattern. Each electrode 12 has a lead wire 14 connected thereto and extending to a terminal 16. One surface of the second glass sheet 6, namely, the surface opposed to the first glass sheet 1 is formed with a common electrode 18. The common electrode 18 also has a lead wire connected thereto and extending to a terminal not shown. The electrodes 12 and 18 are made of a material which has a good adhesion to glass sheets and a high light transmissibility. For example, they are formed of a film of tin oxide.
The liquid crystal display unit shown in FIG. 1 is intended to be a liquid crystal display unit wherein twisted nematic liquid crystal molecules are used in a field effect mode. FIG. 2 diagrammatically shows the cross-sectional construction of such liquid crystal display unit wherein twisted nematic liquid crystal molecules are used in a field effect mode, such a unit being referred to below as a TN-FEM liquid crystal display unit. Referring to FIGS. 1 and 2, the liquid crystal display element 2 is sandwiched between two polarizing sheets 20 and 22. Each of the polarizing sheets 20 and 22 is in the form of a triacetate sheet formed with a film having polarizing characteristics. In FIGS. 1 and 2, the lower surface of the lower polarizing sheet 22 has a reflecting sheet 24 bonded thereto. Therefore, the liquid crystal display unit shown in these figures is of the reflection type. The reflecting sheet 24 may, for example, be an aluminum foil.
FIG. 2 illustrates the various elements on a nonactual scale to give a better understanding of the operation of the TN-FEM liquid crystal display unit. For example, the thickness of the clearance filled with the liquid crystal 10 is shown in an exaggerated manner. In liquid crystal display units of the TN-FEM type, the liquid crystal molecules 26 are disposed in a twisted state. In this state, if voltage is applied between a pair of opposed electrodes 12 and 18, the liquid crystal molecules are lined up in the direction of the electrostatic field, losing their optical activity. FIG. 2 shows a voltage being applied between a pair of electrodes 12 and 18. When the electrostatic field disappears, the liquid crystal molecules 26 return to the twisted state.
Referring to FIG. 2, the principle of operation of the reflection type TN-FEM liquid crystal display unit will now be described. In addition, it is assumed that the respective polarization axes of the polarizing sheets 20 and 22 are at right angles to each other. Unpolarized, external light falls from the top of FIG. 2 upon the polarizing sheet 20, where it is polarized in a first direction. Therefore, the light incident upon the liquid crystal 10 has been polarized in the first direction. If voltage has been applied between a pair of electrodes 12 and 18, then the liquid crystal molecules 26 in the corresponding region are in the illustrated aligned state. Therefore, the light incident upon this portion passes through the liquid crystal 10 and falls upon the lower polarizing sheet 22. The lower polarizing sheet 22 has an axis of polarization extending in a second direction which is at right angles to the first direction. As a result, the light which has been transmitted from the upper polarizing sheet 20 through the liquid crystal 10 is completely cut off. Therefore, no light reaches the reflecting sheet 24 and no reflected light is obtained from the latter, whereby the display looks dark. On the other hand, the light which falls upon other regions than that described above is gradually polarized by the corresponding liquid crystal molecules 26 which have optical activity, with the result that when passing through the liquid crystal 10, the light has been polarized through 90 degrees as compared with the incident light. Thus, the light incident upon the lower polarizing sheet 22 has been polarized in the second direction and passes through the polarizing sheet 22. Therefore, this light reaches the reflecting sheet 24, where it is reflected. The reflected light passes through the polarizing sheet 22 and in the liquid crystal 10 its direction of polarization is restored to the first direction. Therefore, the light passes through the upper polarizing sheet 20. Thus, the display looks bright outside the electrodes 12, 18. On such principle of operation of the display unit, voltage-applied regions look dark, while those having no voltage applied thereto look bright, and the contrast between the dark and bright regions makes the desired display possible.
Attention is invited to the cross-section of the liquid crystal display unit shown in FIG. 2. On the side of the liquid crystal display unit nearer to the viewer, the substantially light transmissible polarizing sheet 20 overlies the first glass sheet 4 which is a flat glass sheet. The surface conditions of the upper surface of the first glass sheet 4 and the lower surface of the polarizing sheet 20 will now be considered. The surfaces of the two sheets must be truly smooth and flat, for, if not, a problem illustrated in FIG. 3 would be caused.
FIG. 3 is a diagram on an enlarged scale for explanation of a problem with conventional liquid crystal display units. Referring to FIG. 3, the laminated state of a first glass sheet 4 and a polarizing sheet 20 is depicted. In such state, if the upper surface of the first glass sheet 1 and the lower surface of the polarizing sheet 20 do not form true planes, a clearance 28 is formed. It will be understood that the presence of such a clearance 28 produces interference fringes known as Newton's rings. Such interference fringes are caused by the fact that an incident light ray 30 to be reflected by the lower surface of the polarizing sheet 20 and another incident light ray 32 to pass through the polarizing sheet 20 and be reflected by the upper surface of the first glass sheet 4 interfere with each other when so reflected. The formation of such interference fringes makes it difficult for the viewer to see the display. Particularly in a display unit adapted to be viewed with the aid of environmental light coming from outside without using a particular light source as in a reflection type liquid crystal display unit, occurrence of interference fringes which hinder viewing, must be prevented.
In order to prevent the formation of the clearance 28 shown in FIG. 3, the upper surface of the first glass sheet 4 and the lower surface of the polarizing sheet 4 must be truly flat. In this case, making the upper surface of the first glass sheet 4 is a machining problem and is not impossible if surface grinding is performed with the greatest care. Making the lower surface of the polarizing sheet 20 truly flat, however, involves not only a machining problem, it is also related to the material of the polarizing sheet 20. For example, if the polarizing sheet 20 contains a resin such as triacetate, problems other than a machining problem one are liable to occur. Triacetate is hygroscopic and expands and contracts to a certain extent, so that it is liable to warp. Therefore, no matter how truly a flat surface may be formed in the machining step, it will eventually be deformed into a curved surface.
FIG. 4 shows the cross-sectional construction of a prior art liquid crystal display unit which is of interest to the present invention. In order to prevent occurrence of interference fringes described above, the polarizing sheet 20 may be spaced apart from the first glass sheet 4 sufficiently to preclude Newton's rings. Referring to FIG, 4, the polarizing sheet 20 is disposed on the first glass sheet 4 through a spacer 36. The spacer 26 is formed, e.g., by punching a plastic film about 0.5 mm thick in a picture frame fashion. This spacer 36 prevents contact between the polarizing sheet 20 and the first glass sheet 4. Therefore, the formation of interference fringes due to Newton's rings is advantageously prevented. In addition, the liquid crystal display unit shown in FIG. 4 is the same as the one shown in FIGS. 1 and 2 except for the arrangement described above. Therefore, like parts are given like reference numerals and a description thereof will be omitted.
The construction shown in FIG. 4 has drawbacks. For example, if such display unit has a small or elongated display face, the spacer 36 can be advantageously used. More particularly, even a spacer having a thickness of only about 0.5 mm is effective to prevent contact between the polarizing sheet 20 and the first glass sheet 4 over the entire area regardless of a possible warp of the polarizing sheet 20. In the case of the display unit having a large display face, however, it is impossible for a spacer about 0.5 mm thick to prevent contact between the polarizing sheet 20 and the first glass sheet 4. Such display unit having a relatively large display face is used, e.g., in a game device shown in FIG. 5.
Referring to FIG. 5, a display face 40 is provided on top of the housing 38 of the game device. The size of the display face 40 could be inferred from the housing 38 measuring, e.g., 5 cm long by 9 cm broad. Thus, it will be understood that the display face 40 shown therein is greater than the display face of at least a wristwatch, a portable small electronic computer or the like. If a liquid crystal display unit having a construction such as shown in FIG. 4 is used to form such display face 40, it is necessary to increase the thickness of the spacer 36 to provide a sufficient clearance between the polarizing sheet 20 and the first glass sheet 4 to ensure that the polarizing sheet 20 will not contact the first glass sheet 4 when warping. However, such increase in the thickness of the spacer 36 is not desirable since it increases the overall thickness of the liquid crystal display unit. It might be also contemplated to increase the thickness of the polarizing sheet 20, but this has the same drawback as described above. Therefore, it is desired to provide a liquid crystal display unit wherein contact between the polarizing sheet 20 and the first glass sheet 4 can be prevented irrespective of the size of the display face.
In addition, it has been understood that interference fringes which must be eliminated are caused by contact between the polarizing sheet and the first glass plate, but their occurrences are not limited to the described particular combination. Generally, interference fringes can occur when two light transmissible sheets are combined. Therefore, any light transmissible sheet placed on a liquid crystal display element involes a similar problem of interference fringes.